Configuration

Configuration

Every Flask app needs settings — database URLs, secret keys, debug modes. Hardcoding these in your app is a bad idea because different environments need different values. Flask's configuration system lets you manage settings cleanly across development, testing, and production.

The config Dictionary

Flask stores configuration in app.config, which behaves like a dictionary. You can set values directly or load them from an object.

app = Flask(__name__)

app.config['SECRET_KEY'] = 'your-secret-key-here'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///dev.db'
app.config['DEBUG'] = True
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

The SECRET_KEY is critical — it's used for signing sessions, cookies, and other security features. Never commit this to version control.

Config Classes

For real apps, organize your settings into classes. Each environment gets its own class with appropriate values.

import os

class Config:
    SECRET_KEY = os.environ.get('SECRET_KEY', 'dev-fallback-key')
    SQLALCHEMY_TRACK_MODIFICATIONS = False

class DevelopmentConfig(Config):
    DEBUG = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///dev.db'

class TestingConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///test.db'

class ProductionConfig(Config):
    DEBUG = False
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL')

config = {
    'development': DevelopmentConfig,
    'testing': TestingConfig,
    'production': ProductionConfig,
    'default': DevelopmentConfig
}

app = Flask(__name__)
app.config.from_object(config[os.environ.get('FLASK_ENV', 'default')])

Now you just set the FLASK_ENV environment variable and the right configuration loads automatically. Development uses SQLite, testing uses a separate test database, and production pulls from environment variables.

Environment Variables

Environment variables are the standard way to handle secrets and environment-specific values. They live outside your code so they never get committed to git.

# Reading environment variables
import os

secret_key = os.environ.get('SECRET_KEY')
database_url = os.environ.get('DATABASE_URL')
debug_mode = os.environ.get('DEBUG', 'False').lower() == 'true'

# Set defaults for development
app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY', 'dev-secret')
app.config['DEBUG'] = os.environ.get('DEBUG', 'True').lower() == 'true'

.env Files with python-dotenv

During development, manually setting environment variables is tedious. The python-dotenv library lets you put them in a .env file that loads automatically.

# Install: pip install python-dotenv

from dotenv import load_dotenv

load_dotenv()  # Loads variables from .env file

app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY')
app.config['SQLALCHEMY_DATABASE_URI'] = os.environ.get('DATABASE_URL')

Your .env file would look like this:

SECRET_KEY=my-super-secret-key
DATABASE_URL=sqlite:///dev.db
DEBUG=True

Make sure .env is in your .gitignore — it contains secrets that should never be in version control.

Key Configuration Values

Every Flask app needs a few essential settings. SECRET_KEY for security, SQLALCHEMY_DATABASE_URI for the database, and DEBUG for development mode. Beyond that, you'll add things like mail server settings, API keys, and file upload limits as your app grows.

Why Configuration Management Matters

Good configuration management means your app works everywhere without code changes. Developers run with debug mode on, testers use a test database, and production uses secure environment variables. It's the difference between an app that "works on my machine" and one that works everywhere.